The present invention relates generally to the art of optical sensors. It finds specific application in conjunction with optical sensors for determining the presence and/or index of refraction of a medium in which the optical sensor may be immersed or placed in contact, and will be described with particular reference thereto. However, it is to be appreciated that the invention will also find further application in other optical devices.
Heretofore, numerous types of optical sensors responsive to the index of refraction have been developed. In such prior sensors, light from a reference light source entered an optical element and was either transmitted into a surrounding medium or internally reflected within the optical element and channeled to a readout apparatus. Frequently, optical fibers provided the channels to direct light from a light source into the optical element and return internally reflected light to the photodetector or other readout device. Although a wide variety of sensors based on this approach have been developed, they have significant disadvantages.
In one prior optical sensor illustrated in U.S. Pat. No. 5,396,325 of Carome and Rainer, the optical element included a flat sapphire plate to which a pair of optical fibers were coupled. The optical fibers were connected to the flat plate at carefully prescribed locations and angles such that light emitted from the first strikes a common central plane and is received at the other, all with prescribed angles of incidence and reflection. One of the disadvantages of this sensor resides in the complexity of manufacture. The attaching of optical fibers at prescribed oblique angles in a spaced relationship to a flat plate is laborious and difficult to mechanize. Moreover, the resultant sensors tended to be relatively delicate. Another disadvantage of this sensor is that increasing the index of refraction of the flat plate does not increase the range of indices that can be sensed.
In another prior art device illustrated in U.S. Pat. No. 4,274,705 of Miller, the optical sensor is a hemi-ellipsoid. The optical fibers are positioned precisely at the two foci of the ellipse. One advantage of the hemi-ellipsoidal construction is that the monitored index is a function of the index of refraction of the optical sensor material. This permits the monitoring of materials with indices of refraction well above the index of refraction of the fiber core. However, the precise placement of the optical fibers at oblique angles against a flat surface is difficult and laborious. Again, the resultant detector tends to be relatively delicate.
In another prior art device described in French Patent No. 2 594 951 to Remouit, the optical sensor is characterized by a single reflection from the measuring surface. An inherent disadvantage of the optical sensor described in the French Patent to Remouit is its lack of sufficient sensitivity to changes in the index of refraction of the substances being monitored.
Another difficulty with many of the prior art optical sensors is that their outputs are not reliable and are difficult to interpret.
The present invention provides a new and improved optical sensor and refractometer arrangement which overcomes the above-referenced problems and others.